I have a group of players with two magically active characters that both use Improved Invisibility on themselves and the rest of the party. I know that it ups the target numbers of the magically active characters but other than that what are good ways to counteract this spell?

Wouldn't dogs work pretty well? Pretty much anything with a keen sense of smell. Also, tripwires and other physical securty. Would thermal pick up someone cloaked under Improved Invisibility?

Some of the options are:

Non visual based sensors (smell, hearing, pressure)

Control of physical access (closed doors, beaded curtains)

Dual natured guards (hell hounds, patrolling spirits, etc)

Wards

Background count (hey, if your office park dosen't need any mages, the ritual sacrifice of a few homeless folks on site will make magic difficult for everybody!)

[edit]

and if you don't like the idea of snatching homeless people off the street and opening their throats, make it a straight up business transaction!

Well, Ms. poor working mom, we see you have way more kids than you can feed. If you are willing to part with a few, we can give you, say, 10,000 Y per child... Just sign here, here, and here, please.
[/edit]

Bob is right, barriers (like doors with beeping keypads), ultrasound, astral perception, pressure plates.
Anything that detects sound, pheromones physical changes. Just pretend you are blind,
and trying to locate someone.

Thermographic will not pick up invis because ir is in the light spectrum.

Drones.

As for thermal, that one's less obvious than it seems—anything that works off of thermovision, like the vision that some metahumans get or that you can tack onto a pair of cybereyes, picks up IR and is thus fooled. Anything that detects heat (most notably the Thermosense Organs in Man and Machine) is not fooled.

As a rule of thumb, if it goes against Signature, it probably bypasses II.

~J

Improved invisibility makes you invisible to electronic sensors as well doesn't it? So you can't be seen by cameras and drones.

No.

Rather, yes, to visual sensors. However, the visual spectrum is one of the least important when talking about vehicle/drone sensors (hence the completely different stat, Signature).

~J

Theres always the good ole spray paint trick for II.

A closed door works wonders at stopping invisible intruders. A closed door with the gaurd's chair right in front of it is even better.